High-temperature operation of gallium oxide memristors up to 600 K

Memristors have attracted much attention for application in neuromorphic devices and brain-inspired computing hardware. Their performance at high temperatures is required to be sufficiently reliable in neuromorphic computing, potential application to power electronics, and the aerospace industry. This work focuses on reduced gallium oxide (GaOx) as a wide bandgap memristive material that is reported to exhibit highly reliable resistive switching operation. We prepared amorphous GaOx films to fabricate Pt/GaOx/indium tin oxide memristors using pulsed laser deposition. Stable resistive switching phenomena were observed in current–voltage properties measured between 300 and 600 K. The conduction mechanism analysis revealed that the resistive switching is caused by the transition between ohmic and space charge limiting current conductions. We elucidated the importance of appropriate control of the density of oxygen vacancies to obtain a high on/off resistance ratio and distinct resistive switching at high temperatures. These results indicate that GaOx is a promising memristor material that can be stably operated even at the record-high temperature of 600 K.


The details of I-V characteristics.
To confirm the endurance characteristics of 100-cycle I-V sweep, the resistive switching behavior of Samples G05, G10, and G15 was characterized at room temperature with a sweep rate of 1.1 V/s and current compliance of −5 mA for 100 cycles in total, as shown in Fig. S1. The first I-V curve at a negative voltage sweep (0→ −3 → 0 V) was performed, and then the subsequent I-V sweep (0 → +1.5 → 0 → −3 → 0 V) was repeated for 100 cycles, as shown in Fig S1. Note that Figs. 2(a)-2(c) are the first 10 cycles of Fig. S1(a)-(c). Consequently, the samples exhibited good endurance through the 100-cycle I-V sweep. We investigate the I-V characteristics starting from the negative and positive voltage applications, as shown in Fig These results indicate that negative voltage application is required for resistive switching. This phenomenon could be related to the high turn-on voltage required for the first SET operation. Once the first negative voltage is applied on the device, the electrical barrier at the Pt/GaOx interface is eliminated by the reduction of GaO x surface via oxygen vacancy supplied from the ITO electrode side, resulting in the evolution of three-order higher current flow and hysteresis I-V characteristics.

Electrode area dependency.
To verify bulk conduction type resistive switching of gallium oxide-based memristors, Aoki et al. and Kura et al. reported a linear relationship between the output current and electrode area [1,2] . Contrary to those reports, the present study did not exhibit clear electrode area dependence of output current, as shown in Fig. S3 for Sample G10 with electrode diameters of 150 and 50 μm. The possible cause is due to the in-plane inhomogeneity of oxygen vacancy distribution. We infer that a non-uniform current path was formed via the high turn-on voltage. Such high turn-on voltage is considered to induce the area-dependent leakage current, e.g., at the edge of the circular electrode, resulting in the in-plane inhomogeneous segregation of oxygen vacancy via resistive switching. Given this assumption, spatially non-uniform distribution of Pt/GaO x Schottky barrier height potentially weakens the area dependency of current flow. To support the non-filamentary conduction mechanism in the present study, we refer to our recent work reported by Masaoka et al. [3] ; a scaling down of output current I against electrode area S was demonstrated via the fabrication of a crossbar array a-GaO x memristors as shown in Fig. S4 (these devices are referred to as Sample GX), where the slopes of double logarithmic plot in I-S characteristic are 0.63 for LRS and 0.65 for HRS. In Masaoka's work, we deposited the a-GaO x films using the same PLD system as the present study and similar deposition conditions with those for Samples G05, G10, and G15. Consequently, the improved scaling-down feature is attributable to the voltage application protocol, where a gradual increase in sweep range could help to avoid the high turn-on voltage and enhance the in-plane homogeneous current flow. A comprehensive investigation remains for future work.

Arrhenius plot of conductivity.
Based on the temperature-dependent resistivity of Samples G05, G10, and G15 [Figs. 3(d)-3(f)], the Arrhenius plot of conductivity σ, the inverse of resistivity, is depicted in Fig. S5. From the slope of the Arrhenius plot from 300 K to 600 K, the activation energy E a of ohmic conduction was extracted for samples G05, G10, and G15: E a = 40, 42, and 40 meV for the LRS and 2, 18, and 31 meV for the HRS. These values compare favorably with 78 meV in a previous study on GaO x memristors [4] , indicating that a similar defect level due to oxygen vacancies was formed in the present study.

Detailed validation of the conduction mechanism
We performed a detailed analysis of the I-V curve based on the SCLC formula [5] . The carrier density n 0 and electron mobility μ n were extracted by fitting with the following formulae.
where q is the elementary charge, d s is the film thickness where SCLC occurs (in this case, the entire film thickness), ε 0 is the permittivity of vacuum, and ε r is the relative permittivity of Ga 2 O 3 . We conducted the curve fitting at LV and HV regions of HRS to obtain the carrier density and electron mobility. Consequently, n 0 = [1.62×10 16 , 1.90×10 16 , and 2.28×10 16 cm −3 ] and μ n = [5.66×10 −2 , 5.31×10 −2 , and 4.82×10 −2 cm 2 V −1 s −1 ] were derived at 300 K for Samples [G05, G10, and G15] (Fig.  S6). Compared to the values n 0 = 2×10 14 cm −3 and μ n = 8 cm 2 V −1 s −1 reported in a previous study of a-GaO x [6] , the carrier density is two orders of magnitude larger, and the electron mobility is two orders of magnitude smaller.